123 research outputs found
Spectrophotometric Resolution of Stellar Atmospheres with Microlensing
Microlensing is a powerful tool for studying stellar atmospheres because as
the source crosses regions of formally infinite magnification (caustics) the
surfaceof the star is resolved, thereby allowing one to measure the radial
intensity profile, both photometrically and spectroscopically. However, caustic
crossing events are relatively rare, and monitoring them requires intensive
application of telescope resources. It is therefore essential that the
observational parameters needed to accurately measure the intensity profile are
quantified. We calculate the expected errors in the recovered radial intensity
profile as a function of the unlensed flux, source radius, spatial resolution
the recovered intensity profile, and caustic crossing time for the two
principle types of caustics: point-mass and binary lenses. We demonstrate that
for both cases there exist simple scaling relations between these parameters
and the resultant errors. We find that the error as a function of the spatial
resolution of the recovered profile, parameterized by the number of radial
bins, increases as , considerably faster than the naive
expectation. Finally, we discuss the relative advantages of binary
caustic-crossing events and point-lens events. Binary events are more common,
easier to plan for, and provide more homogeneous information about the stellar
atmosphere. However, a sub-class of point-mass events with low impact
parameters can provide dramatically more information provided that they can be
recognized in time to initiate observations.Comment: 20 pages, 5 figures, submitted to the Astrophysical Journa
Theoretical Limb Darkening for Pulsating Cepheids
This work presents a new method to compute time and wavelength dependent
center-to-limb brightness distributions for Classical Cepheids. Our model
atmospheres are based on second-order accurate 1-D hydrodynamic calculations,
performed in spherical geometry. The brightness intensity distributions, and
the resulting limb darkening, are computed through the dynamic atmospheres by
using a full set of atomic and molecular opacities. Our results confirm
important differences with respect to equivalent hydrostatic models. The amount
of limb darkening, and the shape of the limb profiles, show a strong dependence
on the pulsational phase of the Cepheid, which cannot be reproduced by static
models. Non-linear effects in our hydrodynamic equations add a new level of
complexity in the wavelength dependence of our limb profiles, which are
affected by the presence of shock-waves traveling through the atmosphere. These
effects, already detectable by present-day interferometers, should be taken
into consideration when deriving limb darkened diameters for nearby Cepheids
with the accuracy required to measure their radial pulsations.Comment: Accepted for publication in the Astrophysical Journa
Principles And Practices Fostering Inclusive Excellence: Lessons From The Howard Hughes Medical Institute’s Capstone Institutions
Best-practices pedagogy in science, technology, engineering, and mathematics (STEM) aims for inclusive excellence that fosters student persistence. This paper describes principles of inclusivity across 11 primarily undergraduate institutions designated as Capstone Awardees in Howard Hughes Medical Institute’s (HHMI) 2012 competition. The Capstones represent a range of institutional missions, student profiles, and geographical locations. Each successfully directed activities toward persistence of STEM students, especially those from traditionally underrepresented groups, through a set of common elements: mentoring programs to build community; research experiences to strengthen scientific skill/identity; attention to quantitative skills; and outreach/bridge programs to broaden the student pool. This paper grounds these program elements in learning theory, emphasizing their essential principles with examples of how they were implemented within institutional contexts. We also describe common assessment approaches that in many cases informed programming and created traction for stakeholder buy-in. The lessons learned from our shared experiences in pursuit of inclusive excellence, including the resources housed on our companion website, can inform others’ efforts to increase access to and persistence in STEM in higher education
Low-Mass Eclipsing Binaries in the Initial Kepler Data Release
We identify 231 objects in the newly released Cycle 0 dataset from the Kepler
Mission as double-eclipse, detached eclipsing binary systems with Teff < 5500 K
and orbital periods shorter than ~32 days. We model each light curve using the
JKTEBOP code with a genetic algorithm to obtain precise values for each system.
We identify 95 new systems with both components below 1.0 M_sun and eclipses of
at least 0.1 magnitudes, suitable for ground-based follow-up. Of these, 14 have
periods less than 1.0 day, 52 have periods between 1.0 and 10.0 days, and 29
have periods greater than 10.0 days. This new sample of main-sequence,
low-mass, double-eclipse, detached eclipsing binary candidates more than
doubles the number of previously known systems, and extends the sample into the
completely heretofore unexplored P > 10.0 day period regime. We find
preliminary evidence from these systems that the radii of low-mass stars in
binary systems decrease with period. This supports the theory that binary
spin-up is the primary cause of inflated radii in low-mass binary systems,
although a full analysis of each system with radial-velocity and multi-color
light curves is needed to fully explore this hypothesis. As well, we present 7
new transiting planet candidates that do not appear among the recently released
list of 706 candidates by the Kepler team, nor in the Kepler False Positive
Catalog, along with several other new and interesting systems. We also present
novel techniques for the identification, period analysis, and modeling of
eclipsing binaries.Comment: 22 pages in emulateapj format. 9 figures, 4 tables, 2 appendices.
Accepted to AJ. Includes a significant addition of new material since last
arXiv submission and an updated method for estimating masses and radi
Field Blue Stragglers and Related Mass Transfer Issues
This chapter contains my impressions and perspectives about the current state
of knowledge about field blue stragglers (FBS) stars, drawn from an extensive
literature that I searched. I conclude my review of issues that attend FBS and
mass transfer, by a brief enumeration of a few mildly disquieting observational
facts.Comment: Chapter 4, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
1, 2, 3, 4: Infusing Quantitative Literacy into Introductory Biology
Biology of the twenty-first century is an increasingly quantitative science. Undergraduate biology education therefore needs to provide opportunities for students to develop fluency in the tools and language of quantitative disciplines. Quantitative literacy (QL) is important for future scientists as well as for citizens, who need to interpret numeric information and data-based claims regarding nearly every aspect of daily life. To address the need for QL in biology education, we incorporated quantitative concepts throughout a semester-long introductory biology course at a large research university. Early in the course, we assessed the quantitative skills that students bring to the introductory biology classroom and found that students had difficulties in performing simple calculations, representing data graphically, and articulating data-driven arguments. In response to students' learning needs, we infused the course with quantitative concepts aligned with the existing course content and learning objectives. The effectiveness of this approach is demonstrated by significant improvement in the quality of students' graphical representations of biological data. Infusing QL in introductory biology presents challenges. Our study, however, supports the conclusion that it is feasible in the context of an existing course, consistent with the goals of college biology education, and promotes students' development of important quantitative skills
Statistical learning techniques applied to epidemiology: a simulated case-control comparison study with logistic regression
<p>Abstract</p> <p>Background</p> <p>When investigating covariate interactions and group associations with standard regression analyses, the relationship between the response variable and exposure may be difficult to characterize. When the relationship is nonlinear, linear modeling techniques do not capture the nonlinear information content. Statistical learning (SL) techniques with kernels are capable of addressing nonlinear problems without making parametric assumptions. However, these techniques do not produce findings relevant for epidemiologic interpretations. A simulated case-control study was used to contrast the information embedding characteristics and separation boundaries produced by a specific SL technique with logistic regression (LR) modeling representing a parametric approach. The SL technique was comprised of a kernel mapping in combination with a perceptron neural network. Because the LR model has an important epidemiologic interpretation, the SL method was modified to produce the analogous interpretation and generate odds ratios for comparison.</p> <p>Results</p> <p>The SL approach is capable of generating odds ratios for main effects and risk factor interactions that better capture nonlinear relationships between exposure variables and outcome in comparison with LR.</p> <p>Conclusions</p> <p>The integration of SL methods in epidemiology may improve both the understanding and interpretation of complex exposure/disease relationships.</p
Direct multi-wavelength limb-darkening measurements of three late-type giants with the Navy Prototype Optical Interferometer
We present direct measurements of the limb-darkened intensity profiles of the
late-type giant stars HR5299, HR7635, and HR8621 obtained with the Navy
Prototype Optical Interferometer (NPOI) at the Lowell Observatory. A triangle
of baselines with lengths of 18.9 m, 22.2 m, and 37.5 m was used. We utilized
squared visibility amplitudes beyond the first minimum, as well as triple
amplitudes and phases in up to 10 spectral channels covering a wavelength range
of ~650 nm to ~850 nm. We find that our data can best be described by
featureless symmetric limb-darkened disk models while uniform disk and fully
darkened disk models can be rejected. We derive high-precision angular
limb-darkened diameters for the three stars of 7.44 mas +/- 0.11 mas, 6.18 mas
+/- 0.07 mas, and 6.94 mas +/- 0.12 mas, respectively. Using the HIPPARCOS
parallaxes, we determine linear limb-darkened radii of 114 R13
R, 56 R4 R, and 98 R9 R,
respectively. We compare our data to a grid of Kurucz stellar model
atmospheres, with them derive the effective temperatures and surface gravities
without additional information, and find agreement with independent estimates
derived from empirical calibrations and bolometric fluxes. This confirms the
consistency of model predictions and direct observations of the limb-darkening
effect.Comment: 13 pages, accepted for publication in A&
Metals detected by ICP/MS in wound tissue of war injuries without fragments in Gaza
<p>Abstract</p> <p>Background</p> <p>The amount and identity of metals incorporated into "weapons without fragments" remain undisclosed to health personnel. This poses a long-term risk of assumption and contributes to additional hazards for victims because of increased difficulties with clinical management. We assessed if there was evidence that metals are embedded in "wounds without fragments" of victims of the Israeli military operations in Gaza in 2006 and 2009.</p> <p>Methods</p> <p>Biopsies of "wounds without fragments" from clinically classified injuries, amputation (A), charred (C), burns (B), multiple piercing wounds by White Phosphorus (WP) (M), were analyzed by ICP/MS for content in 32 metals.</p> <p>Results</p> <p>Toxic and carcinogenic metals were detected in folds over control tissues in wound tissues from all injuries: in A and C wounds (Al, Ti, Cu, Sr, Ba, Co, Hg, V, Cs and Sn), in M wounds (Al, Ti, Cu, Sr, Ba, Co and Hg) and in B wounds (Co, Hg, Cs, and Sn); Pb and U in wounds of all classes; B, As, Mn, Rb, Cd, Cr, Zn in wounds of all classes, but M; Ni was in wounds of class A. Kind and amounts of metals correlate with clinical classification of injuries, exposing a specific metal signature, similar for 2006 and 2009 samples.</p> <p>Conclusions</p> <p>The presence of toxic and carcinogenic metals in wound tissue is indicative of the presence in weapon inducing the injury. Metal contamination of wounds carries unknown long term risks for survivors, and can imply effects on populations from environmental contamination. We discuss remediation strategies, and believe that these data suggest the need for epidemiological and environmental surveys.</p
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